1. Field of the Invention
The present invention is generally related to the measurement of concentrations of analytes in a fluid or gaseous mixture, and more specifically, to a method for making an improved oxygen-sensing optode in which the indicator substance is covalently bonded onto a matrix.
2. Description of the Prior Art
Fiber-optic based oxygen sensing apparatus have proliferated over the years due to their numerous applications in the medical, chemical and environmental fields. Many oxygen sensors rely on the phenomenon of fluorescence quenching as a means for determining the presence of oxygen in a liquid or gaseous mixture. Fluorescence quenching has been a widely used approach for such devices due to the high sensitivity that can be achieved. Systems and instruments implementing fluorescence quenching techniques utilize an encapsulated oxygen-quenchable fluorescence dye that is placed within a gas permeable matrix usually made from a polymer or similar substance. The dye/matrix, called a sensor element or optode, can be applied to the tip of an optical fiber using well-known techniques in the art. A light source with appropriate filtering system provides a select wavelength of light which propagates down the optical fiber and excites the dye. The fluorescence signal, induced by the excitation energy, travels back down the same optical fiber and is collected by a photodetector. The intensity of the fluorescence of the dye, which is a function of the oxygen level in the sample, can be transduced into a partial pressure of oxygen.
While many sensor elements have been developed for use with oxygen measuring devices, there are inherent problems associated with them that are detrimental to the accuracy of the measurements. For example, it is sometimes a difficult task to immobilize the fluorescent dye in a gas permeable matrix because of the chemical and/or physical incompatibility between the dye and matrix. Many of the more widely used oxygen fluorescent dyes are polynuclear aromatic compounds which, because of their high degree of symmetry, usually have low solubility in organic materials. As a result, the fluorescent dyes have a tendency to leach through the permeable matrix into the solution or gas mixture that is being tested.
Various approaches for creating an operable sensor element include absorbing the dye on inorganic or organic solid supports and dispersing the dye in the matrix by way of organic solvents. Many of these techniques still have serious drawbacks if the dye is chemically incompatible with the polymer matrix. These dyes still have a tendency to leach out, particularly when in contact with a sample that includes a substance that has similar properties as the dye/polymer matrix. Unfortunately, such substances include blood proteins and many organic solvents, which are often the samples being tested. As a result of the leaching of the dye during use, the sensing element may have to be continuously replaced to ensure the accuracy of oxygen measurements. Moreover, symmetrical dye molecules that are free to move within a polymer matrix tend to agglomerate which results in changes in fluorescent properties.
Accordingly, those concerned with the development and use of oxygen sensing devices have long recognized the need for an improved method for creating a sensor element that will not leach when placed in the sample solution or during storage and will not suffer dye agglomeration over time. Preferably, the improved method should produce a dye/matrix that can be readily affixed to the end of an optical fiber or other similar device in a single step. Moreover, the sensor made according to the method should be relatively inexpensive to manufacture and should provide accurate oxygen measurements.